Look at Activity Monitor report when it's overloaded to see what's busy.

MacOS and apps have many kinds of background activity which present large transient loads, for example Spotlight indexing and photo library facial scanning.

Do you have a browser or other app infected with a crypto-miner or other malware?

If you create a new user and sign in using only that new user, do you see the same overload?

Are you running an unsupported Mac for Tahoe via OCLP? Thar be performance hazards. Search Macrumors "unsupported" forums.